Dike

Minerals & Geology · Geology & Formation

Definition

A dike is a sheet-like igneous intrusion that cuts across existing rock layers. Dikes form when magma forces its way into a fracture and solidifies, often creating a dark, fine-grained wall that crosscuts lighter host rock. In the field, dikes are useful marker features and can influence where minerals form along the contact between the dike and surrounding rock.

Collectors Context

For collectors, dikes are “map anchors.” Because they cut across bedding, they’re easy to recognize and can help you relocate a productive spot later. Dike contacts can also be interesting zones: heat and fluids associated with intrusion may alter host rock, create baked margins, or open fractures where secondary minerals precipitate. When you find a dike, note its width, rock type (basaltic vs felsic appearance), and contact sharpness. If you’re chasing mineralization, check both margins—sometimes one side is more fractured or altered depending on the host rock. Dikes can be hard and can create steep, blocky exposures, so treat them with basic rockfall awareness when working in cuts or cliffs.

Common Confusions

Dike vs. vein A dike is igneous rock intruded into a crack; a vein is mineral fill. Check texture: igneous grains or glassy margins point to a dike, while a single mineral mass (often quartz/calcite) points to a vein.

Dike vs. sill Dikes cut across bedding; sills run roughly parallel to bedding. If the body crosscuts layers at a steep angle, it’s almost certainly a dike.

Dike vs. fault zone Fault zones may look like linear features, but they show crushing, clay/gouge, and movement indicators. A dike is coherent igneous rock and may show chilled margins against the host.

Dike vs. lava flow Flows are usually layered with flow tops/bottoms and may show vesicles; dikes are tabular bodies that cut existing rock and often maintain thickness across the exposure.

Further Reading

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